Landfills and sewage treatment plants contain siloxanes from many sources.
One source is the semiconductor industry, which produces siloxanes as by-products of reactions involving silicon compound gases. Because siloxanes have detrimental effects on semiconductor products, siloxanes are removed from semiconductor process gases by processes such as adsorption onto diatomaceous earth, silica gel, molecular sieves and activated alumina, which eventually find their way into landfills.
Siloxanes are widely used in detergents as anti-foaming agents, preservatives for concrete and in the personal care industry in products such as deodorants, tooth-pastes, skin care preparations, hair conditioners and anti-perspirants.
The cleaning industry finds many applications for siloxanes. In dry cleaning, siloxanes are used as a more environmentally friendly solvent than traditional chlorofluorocarbons. In the electronics industry, siloxanes are used to clean circuitry.
Siloxanes refer to a class of organosilicon compounds with Si—O—Si linkage. Siloxanes are both cyclic and linear. Cyclic siloxanes have the general formula (R2SiO)n where n equals 3 or greater and R represents an organic ligomer such as for example H or CH3. Linear siloxanes have the formula R3—Si—(OSiR2)n—O—SiR3 where n equals 0 or greater and R represents an organic ligomer such as for example H or CH3. Examples of cyclic and linear siloxanes and their designations are listed in the Table below.
Cyclic siloxanesLinear siloxanesD3: hexamethylcyclotrisiloxaneMM: hexamethyldisiloxaneD4: octamethylcyclotetrasiloxaneMDM: octamethyltrisiloxaneD5: decamethylcyclopentasiloxaneMD2M: decamethyltetrasiloxane
Siloxane-containing waste from industrial and domestic sources such as described above is discharged into landfill sites and sewage treatment plants, along with a variety of biological organic matter.
The organic matter in the waste decomposes to produce biogas containing various volatile organic compounds, such as methane. The biogas can be used to fuel various combustion engines. However, the biogas from landfill sites and sewage treatment plants is contaminated with siloxanes. The siloxane levels in biogas have been growing due to the increased use thereof in numerous products as discussed above. When an engine burns siloxane-contaminated biogas, the siloxane decomposes to form fine silicon dioxide powder. The silicon dioxide powder is deposited on engine parts such as turbine blades, cylinders and heat exchangers. The deposits increase the abrasion of engine surfaces, leading to a loss of engine efficiency and premature engine failure.
Further, as regulations controlling NOx emissions from point-sources become more restrictive, catalytic processes, such as for example selective catalytic reduction, are required to further reduce NOx emissions from engines. These catalysts are readily fouled by the silicon dioxide powder. Such fouling will require that siloxane levels be substantially reduced to low levels, e.g. less than 0.5 ppm, preferably less than 0.05 ppm, and more preferably less than 0.005 ppm.
Various methods have been proposed to remove siloxanes from the biogas streams. Adsorption-based systems are the most common. These systems utilize either a single-pass non-regenerated adsorbent bed or regenerable temperature swing adsorption (TSA) or pressure swing adsorption (PSA) systems, or a hybrid of the two systems. In the absence of moisture, siloxanes are readily adsorbed onto traditional adsorbents, examples of which include, but are not limited to, aluminas, silicas, activated carbons, zeolites and clays. However, at elevated humidity, water dominates the adsorbed phase, resulting in an excessive volume of adsorbent being required for effective siloxane removal. What is more, the presence of other species in the process stream, such as volatile organic compounds (VOC's) and H2S, are also adsorbed by the media and, thus, compete with the siloxanes, further degrading adsorption performance.
Single-pass, non-regenerable adsorption systems are less complex than the regenerable systems, utilizing one or more adsorbent beds through which the biogas is passed. When the adsorbent becomes loaded to its capacity with siloxane, the adsorbent is removed from the system and replaced with fresh adsorbent. These single-pass systems typically use a carbon-based adsorbent, which require frequent change-out due to interference from H2O and H2S, which limit siloxane loading capacity to typically less than 0.05 g siloxane per g carbon. The single-pass, non-regenerable systems have operating costs proportional to the amount of siloxanes in the process stream. For process streams containing high siloxane concentrations, the beds require frequent changeout and replacement, which can be expensive and which restricts the application of these systems to all but the low-siloxane level streams. Additionally, these systems require pretreatment to remove some of the moisture from the process stream, which would otherwise contaminate the adsorbent and considerably shorten adsorbent life.
Regenerable systems using activated alumina, activated alumina plus silica and activated carbon adsorbents to capture the siloxanes have been reported. After the adsorbent is saturated with siloxane impurities, the adsorbed siloxanes are removed in-situ using PSA or TSA to enable the bed to be reused. In the PSA system, the process gas is compressed and passed through one or more of the adsorbent beds where the siloxanes are adsorbed. After the bed is saturated with siloxane, the pressure on the bed is reduced to desorb the siloxanes, which are directed to a waste stream, which is commonly flared along with some of the process gas. PSA systems are effective at removing siloxanes to very low levels but suffer from comparatively high operating costs and lower methane recoveries than TSA units.
In the TSA system, the siloxanes are adsorbed followed by regeneration by heating of the bed. An example of a thermal swing adsorption unit for siloxane removal is disclosed in U.S. Pat. No. 7,306,652, where alumina or alumina plus silica are used to adsorb siloxanes. The saturated adsorbent is regenerated by passing hot air, or biogas at nominally 250° F. through the bed. The hot regeneration gas desorbs the siloxanes, which are then directed to a flare. The regeneration stream can include a slip stream of product gas or external streams used to heat and purge the bed. After heating, the bed is typically cooled, fully or in part, before being placed back on adsorption. Feed and regeneration flow direction are typically in opposite directions though design variations do not require it. The TSA systems are slightly less expensive to operate than the PSA systems, have higher methane recoveries but require pretreatment to remove moisture and other contaminants, such as H2S, in the biogas. The pretreatment steps add additional complexity and increase the operating cost to about that of the PSA system.
One solution to minimize competing water adsorption is to employ hydrophobic adsorbents, such as zeolites, e.g. ZSM-5, zeolite beta and dealuminate zeolite Y. Although effective at minimizing water adsorption at high relative humidity values, hydrophobic zeolites are expensive. Another solution is to employ layered beds. In the layered bed configuration, the inlet layer contains a hydrophilic adsorbent designed to remove water. Examples of hydrophilic adsorbents include zeolite X, aluminum oxide, clays, etc. The outlet layer contains media that targets the removal of siloxanes. The theory of operation is that water is removed up-stream, allowing for the removal of siloxanes from a dry process stream. This solution is not without shortcomings. First, the overall bed size is inordinately large, due to the additional volume required to remove water, leading to increased capital and operating costs. Second, the bed will likely require more frequent regeneration and, due to the increased size, will be more difficult to regenerate.
A typical biogas stream may contain up to and in excess of 50 ppm total siloxane. In addition to the siloxane, the biogas stream will typically be saturated with water vapor, and may contain less than 1% non-methane VOC's plus up to 1,000 ppm of sulfur compounds that include mercaptans, thiols and H2S. The concentration of H2S may be up to and in excess of 500 ppm.
In TSA systems it is sometimes preferred to use the highest temperature possible for both the adsorption operation and desorption operation. Operating the adsorption step at an elevated temperature will reduce the relative humidity and thus the amount of adsorbed water within the pores of the adsorbent. However, operation of the adsorption process at an excessive temperature will decrease the adsorption capacity for the target contaminant. Further, should the contaminant be reactive, it could degrade or react otherwise within the pores of the media, reducing the ability of the media to be regenerated. Conducting the desorption operation at the highest possible temperature will facilitate desorption and reduce the time for desorption and regeneration, thus, increasing process efficiency. When selecting the “highest possible temperature” to conduct the desorption operation, factors including the thermal stability of the media and chemical reactions related to the adsorbed species must be taken into account.
Unfortunately, siloxanes are reactive. Siloxanes are known to undergo both acid and base catalyzed polymerization reactions. In the case of D4 siloxane, while not wishing to be bound by any particular theory, the reaction is expected to proceed as follows:
The hydrolysis product further reacts with the siloxane to yield the polymerized product:

Polymerization of siloxanes will result in the corresponding products being accumulated within the adsorbent pores, thereby reducing adsorbent capacity, as well as the life-time of the adsorbent, leading to frequent change-out and disposal costs being incurred. Elevated desorption temperatures can also increase the polymerization reaction rates, further promoting accumulation of adsorbed species in the pores. As a result of these reactions, adsorption, desorption (or regeneration) temperatures remain relatively low to avoid the undesirable polymerization reactions.
Many commercial adsorbents are prepared with acidic or basic surfaces. For example, during the activation process, functional groups are formed on the surface of carbon. Examples of functional groups associated with the surface of activated carbon include hydroxides, carboxylic acids, ethers and carbonyls. Depending on the method of activation, the carbon is typically acidic or basic. Activation procedures associated with coal and coconut-based carbons typically yield basic media. Activation procedures associated with wood-based carbons typically yield acidic media. The use of commercial adsorbents with acidic or basic surfaces will have the potential to facilitate unwanted siloxane polymerization reactions during TSA processing.